A1081 London Colney Bypass, line source of air and noise pollution. Source: Nigel Cox

A nonpoint source refers to a source of pollution from an origin other than a well defined point. The term is most often applied to air pollution, water pollution and noise pollution. Nonpoint pollution sources may be classified as either line sources, area sources, or less frequently as volume sources. The classic line source is a linear highway, which produces both air pollution and noise pollution. A classic area source is a parcel of land, which produces water pollution in the form of surface runoff containing excessive amounts of nitrates, phosphates, pesticides and herbicides.

The attention to nonpoint sources was heightened, when researchers realized that most human exposure to air and noise pollution derived from nonpoint sources. In the case of noise pollution, over ninety percent of environmental noise exposure worldwide arises from line sources .Less commonly, nonpoint sources of light pollution, thermal pollution and radioactivity are encountered as environmental issues.

Line source air pollution

Interstate 66, Virginia, subject of first U.S. Supreme Court case involving line source air and noise pollution. Source: Creative Commons
While dispersal of air pollutants from point souces has been understood since the 1930s, as outlined by Beychok (2005), the theoretical basis for linear sources of air pollution only became addressed in the late 1960s. The theory of plume rise and Gaussian dispersion developed by Bosanquet, Pearson, Sutton, Pasquill and Briggs is the basic underpinning of line source dispersion, first solved in closed form by Hogan and Venti.

The line source model of Hogan and Venti was first applied to major highways such as the extension of Interstate 66 in northern Virginia and the New Jersey Turnpike widening project, the first case being the initial time an air pollution line source application was included in a U.S. Supreme Court case. Within a decade the air pollution line source model was being applied by most USA state highway departments, the U.S. Federal Aviation Agency and many western European countries.

Hogan and Venti's original model

The line source model (see adjacent equation) was developed to analyze a host of variables, including roadway geometry, wind velocity, turbulence, vehicle mix, emissions library based upon year of analysis, vehicle speed distribution and other parameters.

These line source models have allowed urban planners in the United States to project the future compliance of urban highway construction projects with any relevant state air quality standards as well as with the National Ambient Air Quality Standards (NAAQS).

Line source noise pollution

Line source computer models are used in design of noise abatement as in this sound wall. Source: U.S. DOT
Substantial building of urban highways and transit systems in the 1960s and 1970s generated the demand for understanding the transmission of sound from nonpoint sources. The technical solution is complex, not only due to the linear source geometry, but because the issues of refraction and diffraction must be addressed in the presence of often complex urban terrain and micrometeorology. Refraction can be due to wind shear or lapse rate (vertical temperature gradient near the Earth surface).

By the late 1960s, detailed computer models were developed by ESL Inc. and by BBN, two private firms pioneering this technology. The models were calibrated in a number of major USA highways including U.S. 101 in California, the New Jersey Turnpike, and several major urban highways in Boston, Massachusetts. Not only are such models useful in prediction of sound levels near roadways, but they can be used to design mitigation measures such as sound walls and residential setbacks.

Area source water pollution

Area sources of water pollution began to be recognized as significant in the middle of the twentieth century. The chief sources of such pollution which end up in surface water worldwide are: runoff of excess nitrates and phosphates from agricultural land, runoff of hydrocarbons and lead from parking lots and roadways, feedlots which produce excessive coliform bacteria, surface mining operations and grading operations that generate sediment runoff. Generally these pollutants are deposited on surface soils or pavement, and subsequently are carried by surface runoff to receiving waters.

Some of the area sources are subject to regulation in the USA, such as feedlots, surface mining and some agricultural uses. However, historically and worldwide, these uses still generate massive amounts of water pollution runoff.

Earliest detailed quantification of the area source phenomenon was conducted by the U.S. Environmental Protection Agency and ESL Inc. for instrumented test watersheds in Athens, Georgia.

Other area sources

Other area sources include air pollution sources such as: wildfires; dust from duststorms or agricultural tilling; distributed stack systems such as might be encountered in a large petroleum refinery. Area sources of light pollution might consist of a large illuminated urban area emitting skyglow that could interfere with astronomical observatories. Area sources of radioactivity could include such phenomena as radon gas emission from subsurface rock strata or extensive phosphate rock surface mines, which can often contain considerable amounts of radioactive uranium.

Regulatory basis

The earliest legal framework for analyzing nonpoint sources arose in the USA in the early 1970s. In response to citizen concerns for the burgeoning network of urban highways, the U.S. Congress and various state highway departments began to define nonpoint sources of noise and air pollution from roadways as regulatory targets. In recognition of this focus, air pollution from vehicles began to become defined in the terms of grams per mile, indicating the explicit association of air pollution emissions with a linear source.

Correspondingly, passage of the Clean Water Act (and subsequent amendments) in the USA led to the need to produce computer models of both stationary sources as well as area sources of pollution entering surface waters. Finite difference models were developed to address the transport of soluble and insoluble pollutants as overland flow to reach destination rivers or lakes. The first comprehensive model development and calibration of these models occurred in the USA in the early 1970s. The first basin-wide application was to the Truckee River Basin in California and Nevada, in the USA.; once the models had been calibrated for the Truckee Basin, a tool was available to deduce the relationship of land use and land management decisions to resultant river concentrations to a gamut of pollutants (e.g. pesticides, nitrate, phosphate, sediment). Applications soon followed in other developed countries.